Don't memdup (and leak) in {prefix_}variant_to_gvariant()

The usecase for this function is that we have some pre-existing container that owns the memory and you call something like:

 container_ref(container)
 v = variant_to_gvariant(v, container_unref, container)

In this case we just shouldn't memdup the variant memory, I think that the memdup here is just some accidental copy-pasta that should be removed.

As mentioned in !14 (merged)